Fix kiwix ZIM volume mount to work with a symlink

Bind-mounting data/kiwix/ (the directory) meant a symlink placed
inside it broke: the container has to resolve that symlink itself,
in its own filesystem, where the host path it points to doesn't
exist. Binding the exact data/kiwix/wikipedia.zim file path instead
resolves it on the host at mount time (plain mount(2) semantics), so
the real ZIM can live anywhere on the host without duplicating a
multi-GB file into this repo's data/ directory.

docker-compose.yaml

@@ -60,9 +60,18 @@ services:
   # service's own name), so maptool and kiwix stay independent containers.
   # Keeps restarting (harmlessly) if data/kiwix/wikipedia.zim isn't there
   # yet — GET /api/kiwix just reports unavailable until it comes up.
+  #
+  # The volume binds the exact *file* path, not the data/kiwix/ directory —
+  # deliberately, so data/kiwix/wikipedia.zim can be a symlink to wherever
+  # the (large) real ZIM actually lives. A bind mount resolves a symlink at
+  # its *source* path once, at mount time (plain mount(2) semantics, same
+  # as Podman); a symlink sitting inside an already-mounted directory
+  # doesn't get that treatment — the container would have to resolve it
+  # itself, inside its own filesystem, where the host path it points to
+  # doesn't exist.
   kiwix:
     image: ghcr.io/kiwix/kiwix-serve:latest
     restart: unless-stopped
     command: ["--port=8080", "/data/wikipedia.zim"]
     volumes:
-      - ./data/kiwix:/data
+      - ./data/kiwix/wikipedia.zim:/data/wikipedia.zim